The present invention relates to a magnetic focusing system that uses permanent magnets to focus an electron beam in, for example, a cathode-ray tube.
Both magnetic and electrostatic focusing systems have been employed in cathode-ray tubes (hereinafter referred to as CRTs). Although magnetic systems are more costly than electrostatic systems, when a sharp, bright image is required, as in a projection television set, magnetic Focusing is preferable because of its superior focusing characteristics, and because it is less sensitive to the effects of increased cathode voltage. Hybrid systems comprising an electrostatic prefocusing system and a magnetic main Focusing system have also been used to improve the brightness and definition of both conventional color television and projection television sets.
FIG. 1A shows a frontal view of a conventional magnetic focusing system employing a cast alnico permanent ring magnet 1. FIG. 1B shows a sectional view through line b--b in FIG. 1A. The permanent ring magnet 1 is held between soft iron pole pieces 2a and 2b, which have respective central holes 2c to admit the neck of a CRT. The system is centered on a line that will be referred to as the z-axis. The permanent ring magnet 1 is magnetized parallel to the z-axis, its north pole being in contact with pole piece 2a and its south pole in contact with pole piece 2b. The system also includes a correcting coil 3 and dynamic focusing coil 4, which are wound on a hollow bobbin 5, the inner tubular surface of which is flush with the rims of the central holes 2c.
FIG. 2 illustrates the operation of this magnetic focusing system. The system is placed around the neck 6a of a CRT 6 having a cathode 7 that emits an electron beam 8. Lines of magnetic flux 9 generated by the permanent ring magnet 1 extend from the inside rim of pole piece 2a to the inside rim of pole piece 2b, forming a magnetic lens. An interaction between the beam 8 and magnetic flux 9, which will be described in more detail later, focuses the beam 8 to a spot. A direct current applied to correcting coil 3 adjusts the magnetic flux density so that, when beam 8 is directed down the z-axis, the focused spot falls on the center of the faceplate 6b of the CRT 6, as shown. The beam 8 can be deflected for vertical and horizontal scanning by a deflection yoke 10.
Without further correction, when deflected for scanning, the beam 8 would reach focus on an imaginary spherical surface indicated by the dashed line in FIG. 2, resulting in considerable defocusing of the beam spot on the nearly-flat faceplate 6b. Defocusing would be particularly noticeable at the edges of the screen. The necessary correction is supplied by alternating currents fed to the correcting coil 3 and dynamic focusing coil 4 in synchronization with the vertical and horizontal scanning produced by the deflection yoke 10, a process referred to as dynamic focusing.
FIG. 3 shows circuits typically employed to supply these alternating currents. A voltage waveform synchronized to the horizontal scanning frequency is input at a terminal 11 and passed through a phase corrector 12 to a voltage-to-current converter 13, which feeds current to the dynamic Focusing coil 4. This corrects the defocusing caused by horizontal scanning. A voltage waveform synchronized to the vertical scanning Frequency is input at another terminal 14 and passed through a phase corrector 15 to a voltage-to-current converter 16, which feeds current to the correcting coil 3 to correct the defocusing caused by vertical scanning. This current is superimposed on the direct current applied to the correcting coil 3 to maintain correct focus at the center of the screen.
FIG. 4 shows the flux density distribution of the magnetic lens. The horizontal axis in FIG. 4 is the z-axis, with magnetic flux density B indicated on the vertical axis. The flux density distribution is symmetric about the z-axis, and is maximal in the plane through the center of the permanent ring magnet 1.
The theory of magnetic lenses is well known and has been described, for example, in the book Theory and Design of Electron Beams by J. R. Pierce, published in 1954 by D. van Nostrand Co. (p. 75). Referring to FIG. 5, an electron (e) moving with velocity vector V in a magnetic field with magnetic vector B.sub.n experiences a force that acts at right angles to both B.sub.n and V. (The magnetic vectors of a magnetic field are parallel to its magnetic flux lines.) FIG. 6 shows the trajectory of an electron "a" traveling parallel to the z-axis when it enters a magnetic lens region containing lines of magnetic flux created by a surrounding coil. Because of the relationship shown in FIG. 5, the electron experiences a force in the positive y-direction, which deflects its velocity in that direction. The velocity component in the positive y-direction and the magnetic vector component in the positive z-direction then create a Force acting in the radial direction toward the z-axis, so that the electron spirals in toward the z-axis. As it leaves the magnetic lens region, the electron experiences forces that cause it to spiral in the reverse direction, again toward the z-axis. As a result, the electron is focused to a point "b" on the z-axis. If the magnetic flux density in FIG. 6 is symmetric about the z-axis, then electrons at other points on the incidence plane will experience similar Forces, causing them also to be focused to point "b".
The type of focusing illustrated FIG. 6 applies, for example, in a hybrid Focusing system in which electrostatic prefocusing aligns the electron trajectories parallel to the z-axis. The electron beam velocity is somewhat modulated by electrostatic prefocusing, so it is important for the focal length of the magnetic main lens to be independent of the beam velocity. This condition is satisfied in FIG. 6. Intuitively speaking, the greater the velocity of the incident electron beam, the stronger becomes the force driving it toward the z-axis. Mathematically, the focal length of the magnetic lens is closely related to the rotational period T of an electron about the z-axis, which is given by the equation EQU T=(2.pi.m/e).(1/B)
where m and e are the mass and charge of the electron and B is the magnetic flux density. Note that T does not depend on the velocity of the electron.
In many magnetic focusing systems, the incident electrons do not travel parallel to the z-axis, but diverge from a crossover point. FIG. 7 shows the electron gun of a CRT. The electron gun comprises at least three grids G.sub.1, G.sub.2, and G.sub.3 which are disposed in the neck of the CRT, in Front of the cathode 7. Grid G.sub.1 is biased at a negative voltage with respect to cathode 7, while grids G.sub.2 and G.sub.3 are biased at positive voltages V.sub.2 and V.sub.3 such that V.sub.2 &lt;V.sub.3. The crossover is a point disposed in the area between grids G.sub.1 and G.sub.2 at which the beam is tightly constricted by the electrostatic Fields of these grids. From the crossover point, the beam is accelerated by the potentials of grids G.sub.2 and G.sub.3, and diverges through progressively larger apertures in these grids.
FIG. 8A is a side view of the trajectories of several electrons as they diverge from the crossover point in the electron gun, then are brought to the focal point by an ideal magnetic lens having a constant flux density, with all magnetic flux lines parallel to the z-axis. FIG. 8B shows these trajectories as seen from the focal point; each electron appears to describe a circle, moving first away from, then back to the z-axis. This circular path results from the relations shown earlier. In FIG. 8C, if an electron is moving with a velocity "v" having a positive x-component v.sub.x and positive z-component v.sub.z, the force produced by the positive z-component B.sub.z of the magnetic field will act in the positive y-direction, from below the paper to above the paper in the drawing, as was described in FIG. 5. The motion depicted in FIGS. 8A and 8B is described graphically in FIG. 8D, in which the horizontal axis is the z-axis and the quantities r, B.sub.0, and .theta. are shown on the vertical axis, r being the distance of the electron from the z-axis, B.sub.0 the constant magnetic flux density, and .theta. the angle through which the electron has rotated around one of the circles in FIG. 8B.
Magnetic lenses, like optical lenses, are subject to various types of aberration, including spherical aberration: the tendency of electrons entering the lens at different distances from the z-axis to be brought to focus at different points. Referring to FIG. 9A, the aberration of a magnetic lens depends on its inner diameter "a", its thickness "b," and the beam diameter "r," or the diameter of the neck of the CRT. Increasing "a" in relation to "r" (reducing the ratio r/a) reduces spherical aberration. Increasing the thickness "b" also reduces aberration by making the magnetic flux lines inside the magnetic lens more nearly parallel to the z-axis.
Referring to FIG. 9B, the magnetic flux lines 9 of a magnetic lens are never exactly parallel to the z-axis, but are always curved to a greater or lesser extent. As a result, the magnetic flux density B is not constant but varies as in FIG. 9C, and r and .theta. also vary as in FIG. 9C, rather than as in FIG. 8D. The thickness "b" of the magnetic lens corresponds to the half-width "2d" of the magnetic field, "d" being the distance from the center of the lens, measured along the z-axis, at which the flux density fall to half its maximum value.
From FIGS. 9A and 9B it can be seen that the greater the thickness "b" of a magnetic lens, and the larger its diameter "a" is in relation to "r," the more closely its magnetic flux lines will approximate the ideal case of a uniform magnetic field parallel to the z-axis.
Another important requirement is that the magnetic field generated by the magnetic lens be as symmetrical as possible about the z-axis. Yet another requirement is that the axis of the magnetic lens be aligned with the crossover point of the electron gun. Any asymmetry or misalignment will lead to further lens aberration.
Using a conventional alnico permanent ring magnet, it is difficult to obtain a magnetic lens with satisfactory size, symmetry, and alignment. There are several reasons for this.
An alnico ring magnet is conventionally Fabricated by sand casting, by pouring the molten magnetic material into a mold and allowing it to cool. The cooling rate, however, differs in interior and exterior portions of the mold, creating temperature differences that tend to lead to a non-uniform composition, resulting in loss of symmetry.
A further problem is that remnant oxygen present in the alnico material tends to gasify in the melt, leading to cavities, crystal defects, and cracks, all of which mar the symmetry of the magnetic field generated by the magnet. An alnico ring magnet with a large volume is quite likely to have hidden cavities and cracks in its interior, where they are difficult to detect by inspection.
The alnico magnet that comes out of the mold has a cough and inaccurate surface, which must be ground down to the required dimensions. For alignment and symmetry, it is particularly important to grind the ends of the magnet to a smooth, flat surface, at right angles to the magnet body. The difficulties of producing a large, flat surface by grinding are well known, and the ring shape of the magnet only makes the task harder.
The need to fabricate a new mold whenever the magnet dimensions are changed to accommodate a new CRT design is a further problem. Another problem is the heavy weight of a large alnico ring magnet. The reason that alnico is used despite all these difficulties is that it has good temperature characteristics, as described later.
Another problem with an alnico permanent ring magnet is eddy current loss, which affects dynamic focusing. FIG. 10A shows the position of the dynamic focusing coil 4 in relation to the permanent ring magnet 1. As noted earlier, an alternating current waveform is applied to the dynamic focusing coil 4, to correct for defocusing at the right and left ends of horizontal rasters. This generates a dynamic focusing flux 17, indicated by the symbol o (t).
FIG. 10B shows how the dynamic focusing flux varies in relation to the waveform of the deflection current applied to the horizontal deflection coils. The dynamic focusing flux o (t) is zero at the center of the horizontal deflection current waveform. At other points, the flux o (t) inside the dynamic focusing coil 4 is directed in the negative z-axis direction, so as to weaken the net flux B of the magnetic lens. The current waveform fed to the dynamic focusing coil 4 is parabolic, so that the strength of the flux o (t) and hence the degree of weakening of B increase as the square of the distance from the center of the horizontal scan.
The focal length of the magnetic lens is related to the pitch P given following equation EQU P=P.sub.p x(V.sup.1/2 x(1/B)coso
where K.sub.p is a constant, V is a voltage corresponding to the electron beam velocity, B is the magnetic flux density, and .theta. is the angle between the beam and the z-axis. If B is weakened, then P increases, and with it the focal length. The dynamic focusing flux waveform o (t) in FIG. 10B keeps the beam focused on the faceplate through all parts of the horizontal scan.
Referring again to FIG. 10A, however, the dynamic focusing flux 17 also creates eddy currents 18 on the surface of the permanent ring magnet 1. Flowing around the magnetic ring, these currents give rise to a flux 19 in the direction that tends to cancel the dynamic focusing flux 18. This effect increases the peak value of the current that must be fed to the dynamic focusing coil 4 by a factor of EQU {1+WR/L).sup.2 }.sup.1/2
where W is the number of turns of the dynamic focusing coil 4, R is the reluctance of the closed magnetic circuit created by the eddy currents, and L is the coil inductance. A phase lag of .theta.=tan.sup.-1 (L/RW) also occurs, necessitating a phase correction circuit.
The eddy currents 18 arise from an electromotive force induced by the variation of the dynamic focusing flux o (t) with time, as described by the quantity U=-do (t)/dt, (in units of volts). The eddy current loss (in units of watts) is proportional to the square of the frequency. Multimedia displays and high-definition CRTs require high horizontal scanning frequencies, such as 15.75 kHz, 31.5 kHz, and 33.75 kHz, at which the eddy current loss is appreciable. The conventional permanent ring magnet accordingly requires extra power for dynamic focusing and an extra circuit for phase correction, and as the horizontal scanning frequency off the input video signal increases, the eddy current loss increases in proportion to the square of the frequency.
Various solutions to the foregoing problems have been proposed in the prior art, some of which are illustrated in FIGS. 11 to 14. Elements in these drawings that are equivalent to elements in FIGS. 1A and 1B are indicated by the same reference numerals.
Japanese Patent Application Kokai Publication No. 74344/1989 discloses a permanent ring magnet that is divided into two portions 1a and 1b, which are separated by an iron center yoke 20 as illustrated in FIG. 11. This permits a smaller permanent magnet volume, resulting in Fewer cavities and cracks. However, accurate alignment of the two permanent ring magnets 1a and 1b, center yoke 20, and pole pieces 2a and 2b with respect to the z-axis becomes more difficult. All are likely to be mis-aligned to some extent, with adverse effects on the symmetry and alignment of the magnetic field. To obtain a symmetrical magnetic lens, the above components must have flat surfaces and strictly controlled dimensions, making them difficult and expensive to manufacture. Moreover, this design does not solve the problem of eddy currents.
FIG. 12 shows a variation of the above design disclosed in Japanese Patent Application Kokai Publication No. 60035/1990, using the same reference numerals to denote the permanent ring magnets 1a and 1b and center yoke 20. Lead wires 21 from the correcting coil 3 and dynamic focusing coil 4 are brought out through a hole 22 in pole piece 2a, and a temperature sensor 23 is attached to the center yoke 20, so that the current red to the correcting coil 3 can be adjusted to compensate for the temperature characteristic of the yoke 20. This design also has a case 24 with an inside tube 24a extending through the holes 2c in the pole pieces 2a and 2b and the central hole of the bobbin 5, and an outside cylinder 24b that partly covers the permanent ring magnet 1b and center yoke 20.
One problem with this design is that the hole 22 in pole piece 2a impairs the symmetry of the magnetic focusing Field. Also, although the inside tube 24a aids in positioning the other parts on the z-axis, assembly is inconvenient because it is first necessary to attach the temperature sensor 23 to the center yoke 20, and it is difficult to align the permanent ring magnet 1b and center yoke 20 correctly on the z-axis when they are held by the outer cylinder 24b of the case.
The difficulty of manufacturing a large permanent ring magnet was addressed by Japanese Utility Patent Application Kokai Publication No. 2567/1981. Referring to FIG. 13A, this design employs a large number of small cylindrical rod magnets 1s, which are held between the pole pieces 2a and 2b. The correcting coil 3 and lead wires 21 are as described previously. FIG. 13B shows a perspective drawing of one rod magnet 1s. The rod magnets 1s are disposed in mutual contact with one another as shown in FIG. 13C.
Although the cylindrical rod magnets 1s can be manufactured with comparative ease, once they are assembled in mutual contact as shown in FIG. 13C, they function as a single permanent ring magnet and are still subject to the eddy-current loss described in FIG. 10A, making it necessary to apply extra dynamic focusing current.
Another possible solution to the difficulty of manufacturing a large alnico ring magnet would be to use a ferrite ring magnet instead. Ferrite magnets are made by sintering ferrite powder. Although heavy and not as strongly magnetic as alnico, ferrite magnets are free of cavities and cracks, have a uniform composition, and can be made with good dimensional accuracy. Moreover, their high specific resistance, on the order of 10.sup.10 .OMEGA. cm, reduces the problem of eddy currents.
A problem with using a ferrite magnet, however, is that its magnetic flux density varies with temperature. The temperature coefficient of a ferrite magnet is -0.2%/.degree. C., or about ten times the alnico value of -0.02%/.degree. C. CRTs must operate over a wide temperature range. The operating temperature range at the neck of a CRT is, for example, from 0.degree. C. and 80.degree. C. With a ferrite permanent magnet, temperature variations in this range would cause noticeable changes in focal length. The beam would be in focus only within narrow temperature limits.
To overcome this obstacle to the use of ferrite magnets, Japanese Patent Application Kokai Publication No. 82949/1982 discloses the focusing system shown in FIG. 14, having steel temperature compensation rings 25a and 25b surrounding the ends of a permanent ferrite magnet 1. The permeability of the steel rings 25a and 25b decreases with rising temperature, and their magnetic reluctance increases, so that less magnetic flux can pass through them and more of the magnetic flux must pass through the pole pieces 2a and 2b. This effect compensates for the weakening of the magnetic field generated by the ferrite permanent magnet 1 at higher temperatures.
This technique produces a reasonably flat temperature characteristic in the range from about 10.degree. C. to 50.degree. C., but the characteristic exhibits steep changes at higher and lower temperatures, because of imperfect balance between the temperature characteristics of its different component materials. Focusing performance therefore tends to degrade severely under extreme environmental conditions.
Another difficulty with this design is that, since it performs temperature compensation by controlling external flux leakage, the shape of the temperature characteristic depends strongly on the dimensional accuracy of the permanent magnet 1 and compensation rings 25a and 25b. In practice, the shape of the temperature characteristic tends to be highly variable.
Another method of temperature compensation is to sense the temperature of the ferrite permanent magnet and control the current fed to the correcting coil so as to compensate for the decrease in magnetic flux at higher temperatures, as described in, for example, Japanese Patent Application Kokai Publication Nos. 171040/1986, 256883/1989, and 20174/1990. A difficulty with these schemes is that a ferrite magnet has high specific heat, making it difficult to measure the temperature at the center of the magnet by sensing the temperature at an arbitrary point on its surface. The large thermal inertia of a ferrite permanent magnet also makes it slow to respond to temperature changes, so that focusing characteristics appear to drift with changing temperature.
To summarize the above discussion of the prior art, a magnetic focusing system requires a large, symmetric magnetic lens that is accurately aligned with and centered on the z-axis. If the magnetic lens uses a permanent magnet, to obtain a symmetric lens, the magnet must have a uniform composition and accurate dimensions. If the lens will be used in a CRT with a high horizontal scanning frequency, it should be structured so that eddy currents will not interfere with dynamic focusing. The focal length of the lens should also be insensitive to temperature variations.